As terminals mounted with a wireless LAN interface are becoming widespread, it is becoming important to perform power saving control for terminals operating on a battery as a power supply and the wireless LAN standard provides a power management function to suppress the battery consumption of the terminals (e.g., Non-Patent Document 1). Furthermore, the wireless LAN standard provides radio access control methods such as DCF (Distributed Coordination Function), whereby each access point performs access control in an autonomous and distributed manner, and PCF (Point Coordination Function), whereby a specific station performs access control through polling in a concentrated manner.
The beacon period and DTIM (Delivery Traffic Indication Message) period are provided as general settable parameters of access points. Here, the “beacon period” refers to the period in which a beacon is transmitted. Furthermore, the “DTIM period” refers to the period in which a beacon including information indicating that packet data that has been waiting to be transmitted is transmitted (hereinafter “DTIM beacon”), that is, the period in which a DTIM beacon including information indicating that traffic stored at an access point is transmitted, and is expressed in beacon period units. Furthermore, data transmitted after a DTIM beacon includes a broadcast message or multicast message. When, for example, when the DTIM period is “3,” this means that a DTIM beacon is transmitted once every three beacons. Whether or not a beacon transmitted is a DTIM beacon is determined by whether or not the DTIM count value in a TIM information element, which is an information element included in the beacon transmitted, is “0.”
The beacon period and DTIM period can be changed during operations, but these periods are generally operated at their default values. Furthermore, in areas where wireless LAN is currently widespread, neighboring access points may communicate with each other using the same channel. When neighboring access points carry out transmission using the same channel, interference is avoided by controlling transmission of frames using a collision avoidance mechanism in wireless LAN communication called “CSMA/CA.”
Next, power saving control using DCF will be explained using FIG. 1. FIG. 1 shows an overview of power saving control in an infrastructure mode. In FIG. 1, an access point transmits beacon 10 or DTIM beacon 11 at preset beacon period H1 to inform terminal A and terminal B of the presence of the access point. Beacon 10 and DTIM beacon 11 include a TIM element indicating that frames directed to terminal A and terminal B under the control are accumulated, so that each terminal can recognize that there are frames directed to the terminal, by referring to the TIM information element upon receiving beacon 10 and DTIM beacon 11. Terminal A and terminal B can set two types of states, the “awake” state in which power is supplied to a radio transmitting/receiving section and the “doze” state in which only minimum necessary power is supplied, and can determine the timing to set to the awake state using “Listen Interval” and “Receive DTIMs”. “Listen Interval” defines the period in which terminal A and terminal B receive a beacon, that is, beacon period H1, and “Receive DTIM” indicates whether or not to receive DTIM beacon 11. Since terminal A and terminal B in a power saving mode that receive a broadcast/multicast service must necessarily receive DTIM beacon 11, terminal A and terminal B transition to awake state 15 at DTIM period H2. Furthermore, the access point transmits stored broadcast/multicast frame 12 to terminals under the control immediately after transmitting DTIM beacon 11. By changing the setting to the awake state or the doze state in this way, each terminal can reduce power consumption compared to a case where power is always kept ON. Upon receiving PS-poll 13 transmitted from terminal A and terminal B, the access point transmits unicast data 14 to terminal A or terminal B that transmitted PS-poll 13.
Furthermore, a delay occurs when power consumption is reduced through the power management function of radio terminals as described above, and therefore prior arts are known which attempt to solve the problem of delay by adjusting the DTIM period for applications requiring real-time performance (e.g., Patent Document 1).    Non-Patent Document 1: IEEE Documents Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-128949